Question

State the purpose of the index arm in a stop clock.

Answer 1

Hint:A stop-clock is nothing but a larger stopwatch controlled manually or automatically (sensors), which is used in sports stadiums for viewing at a larger distance.
The index arm in a stopwatch is like the second hand of an analogue clock. It also measures 1min on one complete rotation. So, basically it measures the number of seconds elapsed between two-time intervals.

Complete step by step answer:
A stop-clock is a large analogue time measuring device, which measures the amount of time elapsed between the times when it was started and the time when it was stopped. For example: if the stop clock was started at 2:30:15 (hour: min: sec) and stopped at 2:32:20 then it will show that 2min and 5 secs have elapsed.A stop-clock is nothing but a larger stopwatch controlled manually or automatically (sensors), which is used in sports stadium for viewing at a larger distance. The index arm in a stopwatch is like the second hand of an analogue clock. It also measures one minute in one complete revolution. The only difference is it has a least count of 0.2s usually, while normal analogue watches have a least count of 1s. This means that the index arm ticks more than one time in a second, like 5 times for $\dfrac{1}{5} = 0.2$ least count. The purpose of the index arm is to measure the number of seconds elapsed.

Note: There is one more hand (dial) in a stopwatch, which is used to measure the number of minutes elapsed from the time stopwatch was started to the time when it was stopped. Similarly, the index arm is used to measure the number of seconds elapsed, as explained above. So, if we have to read total time elapsed in an analogue stopwatch, we have to read on two dials; first the number of minutes elapsed (for example 3mins) and then number of seconds elapsed (for example 14.4secs), so the total reading will be 2mins and 14.4secs.